1. Technical Field
The present disclosure relates to optical apparatus and methods of using same.
2. Description of the Background Art
The conventional apparatus in the market for photomask inspection generally employ ultra-violet (UV) light with wavelengths at or above 193 nanometers (nm). This is suitable for masks designed for use in lithography based on 193 nm light. To improve further the printing of minimum feature sizes, next generation lithographic equipment is now designed for operation in the neighborhood of 13.5 nm. Accordingly, patterned masks designed for operation near 13 nm must be inspected. Such masks are reflective, having a patterned absorber layer over a resonantly-reflecting substrate (EUV multilayer, typically 40 pairs of MoSi with a 7 nm period. The conventional inspection apparatus uses optics with a combination of wavelength and numerical apertures (NA) that are not sufficient (i.e. too small) to resolve pattern features and pattern defects of interest (printable) in EUV mask patterns characterized by a half-pitch below 22 nanometers (nm).